This invention relates to power inductors and transformers, and more particularly to high frequency power inductive devices including a ferrite magnetic core structure with an improved air gap arrangement to result in low acoustic noise and reduced fringing flux.
Air gaps are needed in many power magnetics to reduce the permeability to prevent saturation of the core due to dc components of current or, as in inductors, due to high values of alternating current. Conventional three-legged and two-legged magnetic cores such as are shown in FIGS. 1 and 2 are made in two parts, and the air gap is established in series between the two halves of each winding leg extending in the direction of the length of the winding leg. Due to the magnetic field across the air gap, forces exist which tend to periodically shorten the air gap. Thus, movement and flexing of the two parts, especially at high flux levels, in many cases produces a considerable amount of acoustic noise. Additionally, fringing flux from the air gap causes eddy current heating of the windings in its vicinity as well as heating of any metallic bands used to clamp the core parts together. These problems are accentuated at high frequencies of about 5-20 kHz, and it may be necessary to employ nylon tape or plastic fixtures with nylon bolts to clamp the core parts.
Magnetic material with low permeability has also been used in the fabrication of core structures so that no large air gaps need exist. These materials are made by diluting conventional magnetic material with a binder which results in essentially a continuous or distributed air gap. However, these cores are always larger than cores with conventional air gaps due to the diluted nature of the magnetic material. That is, for a given flux level the diluted material requires a larger area than conventional material if the flux density is to be the same in both cases, and the watts per pound figure should always be higher for the diluted material.
Ferrite magnetic material is gaining widespread use in high frequency power conversion circuits and, in addition to its high permeability and high resistivity, has a unique property in that it can be machined or pressed into various geometrical shapes. Thus, unconventional means can be used to realize air gaps in power magnetics. The present invention is directed to improved low acoustic noise power inductive devices achieved by magnetic cores incorporating improved air gap configurations, and to such devices with low air gap fringing flux.